FIG. 4 illustrates a tapered roller bearing 1 that is assembled in rotation support sections of rotary machinery such as machine tools, industrial machinery and the like to which large radial loads and thrust loads are applied. A tapered roller bearing 1 includes an outer ring 2, an inner ring 3 that is arranged concentrically with the outer ring 2, plural tapered rollers 4, and a retainer 5. The outer ring 2 has an outer-ring raceway 6 having a partial cone shaped surface around the inner-circumferential surface. The inner ring 3 is arranged on the inner-diameter side of the outer ring 2, and has an inner-ring raceway 7 having a partial cone shaped surface around the outer-circumferential surface. A large-diameter-side rim section 8 is formed around the end section on the large-diameter side (right-end section in FIG. 4) of the outer-circumferential surface of the inner ring 3 so as to protrude outward in the radial direction from the inner-ring raceway 7, and a small-diameter-side rim section 9 is formed around the end section on the small-diameter side (left-end section in FIG. 4) of the outer-circumferential surface of the inner ring 3 so as to protrude outward in the radial direction from the inner-ring raceway 7. The tapered rollers 4 are rotatably arranged between the outer-ring raceway 6 and inner-ring raceway 7. The tapered rollers 4 are shaped such that the diameter (outer diameter) of the outer-circumferential surface, which is the rolling contact surface, gradually becomes larger going in the direction from the end section on the small-diameter side (left-end section in FIG. 4) toward the end section on the large-diameter side (right-end section in FIG. 4). The end surface 10 on the large-diameter side of a tapered roller 4 (right-end surface in FIG. 4) comes in sliding contact with the inside surface 11 of the large-diameter-side rim section 8, and the end surface 12 on the small-diameter side (left-end surface in FIG. 4) faces the inside surface 13 of the small-diameter-side rim section 9 by way of a gap. The tapered rollers 4 are rotatably held by the retainer 5.
FIGS. 5A to 5D illustrate an example of a method for manufacturing a tapered roller. First, using the method such as disclosed in JP H09-076029 (A), JP 2008-114238 (A), a truncated cone shaped preliminary intermediate raw material 14 as illustrated in FIG. 5A is obtained by performing plastic working such as a forging process of a circular column shaped material that is made of a metal such as bearing steel. Next, as illustrated in FIG. 5B, with the large-diameter-side end surface 10 of the preliminary intermediate raw material 14 as a reference, grinding (rough grinding) using a grindstone 16 having large abrasive grain is performed on the outer-circumferential surface 15 of the preliminary intermediate raw material 14 that will become the rolling contact surface that comes in rolling contact with the outer-ring raceway 6 and inner-ring raceway 7 in the completed state. More specifically, with the center axes of the preliminary intermediate raw material 14 and the rotating shaft 19 of a spindle device 18 aligned, the large-diameter-side end surface 10 of the preliminary intermediate raw material 14 is butted against the tip-end surface of the rotating shaft 19 (bottom-end surface in FIG. 5B). In this state, as the preliminary intermediate raw material 14 is rotated by rotating the rotating shaft 19, the grindstone 16 is brought into contact with the outer-circumferential surface 15 of the preliminary intermediate raw material 14, and an intermediate raw material 17 is obtained by performing a grinding process on the outer circumferential surface 15.
Next, as illustrated in FIG. 5C, a grinding process is performed on the large-diameter-side end surface 10 of the intermediate raw material 17 to make the length (dimension in the axial direction) L of the intermediate raw material 17 a pre-determined specified value Lc. More specifically, the center axes of the intermediate raw material 17 and the rotating shaft 19a of the spindle device 18a are aligned, then the small-diameter-side end surface 12 of the intermediate raw material 17 is butted against the tip-end surface of the rotating shaft 19a, and the intermediate raw material 17 is rotated by rotating the rotating shaft 19a. In this state, while measuring the length L of the intermediate raw material 17 in an in-process, a cup-type grindstone 20 is brought into contact with the large-diameter-side end surface 10 of the intermediate raw material 17, and a grinding process is performed on the large-diameter-side end surface 10 by causing the grindstone 20 to displace a specified amount in the axial direction of the intermediate raw material 17. By completing the grinding process at the instant that the length L of the intermediate raw material 17 has reached a specified value Lc, a final intermediate raw material 21 as illustrated in FIG. 5D is obtained. The grindstone is not limited to being a cup-type grindstone as illustrated in the figure, and it is possible to use grindstones having various kinds of construction, such as a flat-shaped grindstone. The large-diameter-side end surface 10 of the intermediate raw material 17 can also be a partial spherical surface such as illustrated in FIG. 6. In that case, the length L of the intermediate raw material 17 is the length between the apex of the large-diameter-side end surface 10 and the small-diameter-side end surface 12. In any case, after the final intermediate raw material 21 has been obtained, a grinding process (finish grinding using superfinishing) is performed on the outer-circumferential surface 15 of the final intermediate raw material 21 using a grindstone that has small abrasive grain to obtain a tapered roller 4. In order to reduce the sliding resistance between the large-diameter-side end surface 10 of the tapered roller 4 and the inside surface 11 of the large-diameter-side rim section 8 of the inner ring 3, a grinding process using a grindstone is further performed on the large-diameter-side end surface 10 of the tapered roller 4 using a method such as disclosed in JP 2011-152597 (A).
In the case of this kind of method for manufacturing a tapered roller, there is a possibility of variation in the shape of the outer-circumferential surface 15 of the final intermediate raw material 21 that is defined by the outer diameter D of the outer-circumferential surface 15 when the position P in the axial direction based on the outer-diameter-side end surface 10 is the same. In other words, a preliminary intermediate raw material 14 that has a truncated cone shape is obtained by performed plastic working on a circular column-shaped metal raw material, so a certain amount of variation in the length (dimension in the axial direction) Lo (see FIG. 5A) of the preliminary intermediate raw material 14 will occur due to unavoidable manufacturing error and cannot be avoided. In the step of obtaining an intermediate raw material 17 by performing a grinding process on the outer-circumferential surface 15 of the preliminary intermediate raw material 14 with the large-diameter-side end surface 10 as a reference, there is very little variation in the shape of the outer-circumferential surface 15 of the intermediate raw material 17. However, the grinding process when performing a grinding process on the large-diameter-side end surface 10 in order to regulate the length (dimension in the axial direction) L of the intermediate raw material 17 to the specified length Lc is performed with the small-diameter-side end surface 12 of the intermediate raw material 17 as a reference, and the amount of grinding during the grinding process (feed amount of the grindstone 20) may vary due to variation in the length Lo of the preliminary intermediate raw material 14, that will cause variation to occur in the shape of the outer-circumferential surface 15 (outer diameter D of the outer-circumferential surface 15) of the final intermediate raw material 21. For example, when the cone angle of the tapered roller 4 (two times the angle value of the outer-circumferential surface 15 (angle between the center axis of the tapered roller 4 and the generating line)) is taken to be 4 degrees, and the variation in the length in the axial direction of the preliminary intermediate raw material 14 is taken to be 500 μm, the variation in the outer diameter D of the outer-circumferential surface 15 of the final intermediate raw material 21 becomes about 35 μm, which is too large to be ignored. Therefore, when plural tapered rollers 4 that were obtained by performing a finishing process on final intermediate raw materials 21 that were made by the method for manufacturing tapered rollers illustrated in FIG. 5A to FIG. 5D are assembled in a tapered-roller bearing 1, there is a possibility that variation will occur in the contact bearing pressure at the areas of rolling contact between the rolling contact surfaces of the tapered rollers 4 and the outer-ring raceway 6 and the inner-ring raceway 7, and there is a possibility that vibration and noise that occur during operation of the tapered-roller bearing 1 will become large. When trying to regulate the variation in the outer diameter D of the outer-circumferential surface 15 of the final intermediate raw material 21, there is a possibility that the amount of processing during finish grinding will increase, or that the processing time will vary, causing an increase in the manufacturing cost of the tapered rollers 4.
In the process illustrated in FIG. 5C, a method of grinding the large-diameter-side end surface 10 of the intermediate raw material 17 with the grindstone 20 for pre-determined amount of time is also possible. However, in that case, there is a possibility that variation in the amount of grinding (feed amount of the grindstone 20) will occur due to the state of the large-diameter-side end surface 10, and that variation in the outer diameter D of the outer-circumferential surface 15 of the final intermediate raw material 21 and also in the outer diameter of the rolling contact surface of the tapered rollers 4 will be too large to be ignored.